Recently, a water absorbent resin is widely used as a main construction material of sanitary materials (absorbent articles) such as paper diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids (e.g. urine, blood, and the like).
Well-known examples of the water absorbent resin are (i) cross-linked partially neutralized polyacrylic acid; (ii) a hydrolyzed starch-acrylonitrile graft polymer; (iii) a neutralized starch-acrylic graft polymer; (iv) a saponified vinyl acetate-acrylic ester copolymer; (v) cross-linked carboxymethylcellulose; (vi) hydrolyzed acrylonitrile copolymer or hydrolyzed acrylamide copolymer, or cross-linked acrylonitrile copolymer or cross-linked acrylamide copolymer; (vii) a cross-linked cationic monomer, (viii) a cross-linked isobutylene-maleic acid copolymer; (ix) a cross-linked body of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid; (x) and the like. In this manner, the water absorbent resin is a hydrophilic resin which is insolubilized due to its evenly cross-linked structure inside a polymer.
Incidentally, there has conventionally been needs for a water absorbent resin having the following water absorbent properties: (i) a high absorbency for a aqueous liquid such as a body fluid, (ii) an excellent absorption rate, (iii) excellent liquid permeability, and (iv) excellent gel strength of a swollen gel, and (v) an excellent absorptive capacity when water is absorbed from a base material containing a aqueous liquid, (vi) and the like.
Thus, in order to attain the foregoing absorbing properties, usually, surfaces of particles of the water absorbent resin are further cross-linked by using a cross-linking agent or the like, thereby causing the particles to have a cross-linking density gradient. Thus, (i) a water-absorption rate of the water absorbent resin is improved, (ii) generation of fish eye is prevented, (iii) gel strength is improved, (iv) an absorbency of the water absorbent resin under pressure is improved, (v) gel blocking is prevented, and (vi) liquid permeability is improved.
For example, surface cross-linking processes for causing a vicinity of particle surfaces of the water absorbent resin to have a cross-linking density gradient are described in Patent Document 1 (European Patent No. 0349240), Patent Document 2 (European Patent No. 0605150), Patent Document 3 (Japanese Publication for Unexamined Patent Application, Tokukaihei 7-242709), Patent Document 4 (Japanese Publication for Unexamined Patent Application, Tokukaihei 7-224304), Patent Document 5 (U.S. Pat. No. 5,409,771), Patent Document 6 (U.S. Pat. No. 5,597,873), Patent Document 7 (U.S. Pat. No. 5,385,983), and the like. In addition to the foregoing methods recited in the patent Documents, a water absorbent including a water absorbent resin and metal soap in order to improve liquid permeability, is described in Patent Document 8 (Japanese Publication for Unexamined Patent Application, Tokukaisho 61-58658).
Moreover, there are needs for such a water absorbent resin which not only has the foregoing water absorbent properties, but also has the following advantages: (i) The water absorbent resin has excellent fluidity at the time of production and transportation of the water absorbent resin, at the time of production of an absorber by processing the water absorbent resin and a fiber base material or the like, and at the time of moisture absorption, so that the water absorbent resin rarely adheres to an apparatus or the like; and (ii) The water absorbent resin is not significantly deteriorated in terms of water absorbent properties, when subjected to a mechanical shock. As an attempt to produce water absorbent resin having excellent fluidity at the time of moisture absorption, a water absorbent in which an inorganic substance such as amorphous silicon dioxide, kaoline, or the like is added, is proposed. Specifically, for example, art related to a water absorbent including powder of an inorganic substance and powder of a water absorbent resin is disclosed in Patent Document 9 (U.S. Pat. No. 4,734,478), Patent Document 10 (Japanese Publication for Unexamined Patent Application, Tokukaisho 59-80458), and Patent Document 11 (U.S. Pat. No. 5,453,323).
Further, for example, a water absorbent in which stearic acid and powder of an inorganic substance are added as additives is described in Patent Document 12 (Japanese Publication for Unexamined Patent Application, Tokukaisho 63-105064), and a water absorbent in which quaternary ammonium salt is added as additives is described in Patent Document 13 (U.S. Pat. No. 5,728,742). Moreover, a water absorbent in which oxalic acid (salt) and a multivalent metal compound such as (i) metal oxide such as silicon oxide or the like, (ii) metal sulfate such as calcium sulfate or the like, or (iii) the like are added, is described in Patent Document 14 (Japanese Publication for Unexamined Patent Application, Tokukaihei 7-228788).
Moreover, a water absorbent resin compound in which polyethyleneglycol, polypropyleneglycol, or the like are added is disclosed in Patent Document 16 (European Patent No. 0001706).
However, the water absorbents recited in the foregoing Patent Documents have the following various problems. That is, as to the water absorbent resins recited in the Patent Documents 1 to 8, the fluidity at the time of moisture absorption is insufficient. Further, as to the water absorbents recited in the Patent Documents 9 to 11, an inorganic powder is used in order to improve the fluidity at the time of moisture absorption, so that the water absorbent properties are deteriorated due to the hardness of the inorganic substance when the water absorbent resin is subjected to a mechanical shock (damage). Therefore, absorbent articles using the water absorbents recited in the Patent Documents 9 to 11 cannot attain sufficient absorbing properties.
Further, as to the water absorbents recited in the Patent Documents 12 and 13, there is a problem of safety because there is a possibility that, when the water absorbent is used as an absorbent article, the additive contained in the water absorbent liquates out into an aqueous liquid such as urine and the like absorbed by the water absorbent. Therefore, in the case of using the water absorbents recited in the Patent Documents 12 and 13 as a material for a paper diaper or the like for example, a aqueous liquid such as urine is hard to diffuse in the whole water absorbent. As a result, a return amount of the aqueous liquid absorbed by the water-absorbent provided in the paper diaper is increased, so that the water absorbent properties are deteriorated.
Further, as to the water absorbent recited in the Patent Document 14, the oxalic acid (salt) is contained as the additive, so that there is a problem of safety concerning the oxalic acid. Moreover, when the water absorbent resin is subjected to a mechanical shock, it is difficult to alleviate (absorb) the mechanical shock, and the water absorbent resin is even damaged, so that the water absorbent properties are significantly deteriorated. This is because the oxalic acid (salt) and an inorganic substance such as a multivalent metal compound contained as the additive are hard.
Further, in the case of using the inorganic substance as described above, the fluidity at the time of moisture absorption is improved, but there is the following problem: powder fluidity of the water absorbent is deteriorated under such a dry condition that a moisture content is less than 10 mass %.
On the other hand, when the water absorbents recited in the Patent Documents 15 and 16 are used, it is possible to slightly reduce frictional resistance of the water absorbent. However, also in the water absorbents recited in the Patent Documents 15 and 16, the powder fluidity (anti-caking) at the time of moisture fluidity and the powder fluidity under such a dry condition that the moisture content is less than 20 mass %, particularly less than 10 mass %, are insufficient. At the time of moisture absorption, viscosity occurs among particles, so that this results in blocking or caking. As a result, the powder fluidity is deteriorated. Further, the particles themselves have a high frictional coefficient also in a dry state. Therefore, in the water absorbent under the dry condition, the frictional resistance is increased, so that it is difficult to smoothly transport and carry the water absorbent at the time of production or the like of the water absorbent.